1. SPIM and MIPS programming

2. Review: Memory Organization
Large single-dimension array
A memory address is an index into the array
Memory stores 8 bits in each address
Registers store 32 bits of information
For example:
Li $t0, 0
lw $t0, 0($t1)
Then $t0 will contain the data of the first four memory locations 6 5 4 3 2 1
8 bits of data

3. Review: Registers MIPS has 32 registers, each containing 32 bits
Some registers are listed below
$zero: constant 0
$at: reserved for the assembler
$v0 - $v1: function results
$a0 - $a3: function arguments
$t0 - $t9: temporary
$s0 - $s7: temporary

4. Arithmetic Instructions (1/2)
Some instructions include:
Add
Subtract
Multiply
Divide
Must break expression into simple operations
use temporary registers ($t0 to $t9) to store intermediate results
use load to insert a value into a variable
use store to assign a value to a variable

5. Instructions Instruction Meaning add $s1, $s2, $s3 $s1 = $s2 + $s3
sub $s1, $s2, $s3 $s1 = $s2 – $s3
lw $s1, 100($s2) $s1 = Memory[$s2+100]
sw $s1, 100($s2) Memory[$s2+100] = $s1

6. Global variables C program Assembly Program int variable = -42; .data
variable: .word -42
int r1; r1: .space 4
.space allocates empty space in bytes
.word allocates 4-byte value followed by the value to store

7. Assember Directives
All assembler directives have names that begin with a period (`.').
Examples:
.word w1
allocate a 4-byte word
.half h1
allocate 2-bytes
.byte b1
allocate a single byte
.ascii “hello"
allocate a sequence of bytes and store ASCII values
.asciiz “hello"
allocate a sequence of bytes and store ASCII values. terminate string with 0 byte (end of string)
.float
.double

8. Input/Output
On real machines (including real MIPS computers), I/O is very complicated
usually handled by operating system
In SPIM, I/O is managed by system calls
syscall instruction
SPIM suspends simulation to perform I/O, then resumes simulation

9. System calls Systems calls are used to interact with the user
To make a system call
determine which service you want
put service’s call code in register $v0
put arguments in registers $a0, $a1, etc.
execute the syscall instruction
results are in registers $v0, $v1

10. System calls

11. Input/Output: Numbers
.data
value: .space 4
.text
main:
li $v0, 5 #Syscall 5: read int
syscall
sw $v0, value #Number is in $v0
li $v0, 1 #Syscall 1: print int
lw $t0, value
mul $a0, $t0, $t0 #$a0 is what to print
li $v0, 10 #Exit

12. Input/Output: strings
.data
msg: .asciiz "Type a string: "
buff: .space 20
.text
main:
li $v0, 4 # print string
la $a0, msg # $a0: address of string
syscall
li $v0, 8 # read string
la $a0, buff # $a0: max length of buffer
li $a1, 20 # $a1: max length of string
li $v0, 4
la $a0, buff
li $v0, 10 # exit